This invention relates to devices for changing the compression ratio of internal combustion engines, by varying the volume of a combustion chamber assumed when the piston is in the top dead center (TDC) position.
A compression ratio-changing device for an internal combustion engine is known, e.g. by Japanese Provisional Patent Publication (Kokai) No. 58-91340, which comprises an eccentric bearing interposed between the piston and the connecting rod such that the axial position of the piston relative to the connecting rod can be changed with a change in the angular position of the eccentric bearing. The device further comprises a hudraulically-operated lock pin arranged within the connecting rod for being pushed into and moved from the eccentric bearing by means of hydraulic oil pressure applied thereto so as to cause the eccentric bearing to be locked to and unlocked from the connecting rod, thereby changing the compression ratio of the engine.
However, according the prior art device, hydraulic oil pressure applied to the lock pin is supplied from two independent main oil passages, one for setting a higher compression ratio and the other for setting a lower compression ratio, formed in the cylinder block by way of respective oil passages extending through the crankshaft, crank pin, and connecting rod. These last-mentioned oil passages for feeding hydraulic oil also serving as lubricating oil require spacing for formation thereof within bearings provided in the crankshaft, crank pin, etc. Since the bearings are each disposed within a limited space, it is difficult to obtain the spacing within the bearings for formation of the dual-purpose oil passages for changing the compression ratio and lubricating the bearings. Furthermore, the aforementioned two main oil passages are selected by means of a changeover valve arranged within the cylinder block at a location upstream of the main oil passages, in other words, commands for bringing the lock pin into and out of its locking position are issued at a location considerably remote from the lock pin and transmitted to the lock pin through the long oil passageways. As a consequence, the lock pin does not move in quick response to the commands, resulting in low responsiveness in changing the compression ratio.
Another compression ratio-changing device has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 54-106724, which has a hydraulic pressure chamber defined between an upper inner end face of the piston and an opposed outer end face of the piston guide which is secured to the connecting rod and axially slidably received within the piston. The piston is axially displaced relative to the piston guide by applying thereto hydraulic pressure within the hydraulic pressure chamber which is supplied from a hydraulic pressure control device provided on the cylinder block side, thereby changing the volume of the combustion chamber and hence the compression ratio of the engine.
However, according to the proposed device, the hydraulic pressure for controlling the compression ratio is supplied from the hydraulic pressure control device to the hydraulic pressure chamber by way of oil passages formed through the crankshaft, crank pin and connecting rod as well, thereby unavoidably requiring spacing within bearings of the crankshaft and the crank pin for providing the oil passages for the purpose of control of the compression ratio. In order to not only obtain spacing for the oil passages for controlling the compression ratio, but also secure lubrication of the bearings, there may be supposed two methods. That is, the first method is to provide an exclusive oil passageway for controlling the compression ratio in addition to the lubricating oil passageway, while the second method is to provide a dual-purpose oil passageway for feeding hydraulic oil for controlling the compression ratio as well as lubricating the bearings, the pressure of hydraulic oil being set to values within such a range that the hydraulic pressure can always serve to lubricate the bearings, irrespective of whether it is set to a higher value for higher compression ratio or to a lower value for lower compression ratio. However, according to the former method, it is difficult to form the exclusive oil passageway within limited spaces in the bearings. According to the latter method, on the other hand, the lower hydraulic pressure value for obtaining the lower compression ratio cannot be set to a value low enough to appropriately control the compression ratio because such a low pressure value is too low for lubrication, and if the lower pressure value is set to a value higher than such a low value, the higher hydraulic pressure value will correspondingly be excessively high, thereby necessitating increasing the capacity of the hydraulic pressure control device or the mass or weight of the piston. Further, in this prior art device, the hydraulic pressure supplied to the hydraulic pressure chamber is controlled by the hydraulic pressure control device located remotely from the hydraulic pressure chamber, thus resulting in difficulty to obtain quick displacement of the piston relative to the piston pin and hence low responsiveness in changing the compression ratio of the engine.